Local Controller described a go around event detected by an ASDE-X alarm involving an unauthorized runway entry by a vehicle; one of several operating on the airport during an emergency.

Narrative

While on the Local Control position; an aircraft on final approach executed; on their own; a go around due to an equipment malfunction. This aircraft was later declared an emergency aircraft due to a hydraulic problem. While this emergency aircraft was preparing for a second approach to land; coordination was being conducted to obtain the use of Runway XXR. Several vehicles needed to remove markers and clear the intended runway for use prior to the aircraft arrival. Also emergency vehicles were driving out from their respective locations in order to respond to the emergency. Meanwhile; aircraft were still landing and departing Runways XXL; YYR; and Runway YYL. Runway XXR was inspected and reopened a few minutes later. A B767 first checked on frequency about 8 miles on final to land on Runway XXL. The runway was visually inspected to be free and clear for use; and the B767 was cleared to land on Runway XXL. With the B767 on an approximately 2 mile final; I used binoculars and inspected to see that the gear was in the down position. Also; the emergency aircraft was concurrently on a 7 mile final to Runway XXR; so I moved my attention towards the emergency aircraft and visually inspected with the binoculars that their gear also appeared to be in the down position. With the B767 now on a short final to Runway XXL and beginning to flare out; the ASDE-X (surface detection equipment) alarmed and alerted that Runway XXL was occupied and instructed to send the aircraft around. I then looked at the approach end of the runway and observed an unauthorized vehicle at the approach end of Runway XXL; and immediately responded to the ASDE-X alert and instructed the B767 to 'go around'. The pilot then read back 'go around'; and added that they too saw the vehicle encroach on to the active runway. The B767 was then appropriately vectored and re-sequenced for an arrival back to Runway XXL at which they later landed safely. The emergency aircraft also landed safely on Runway XXR without any further incident. It is still undetermined which vehicle is responsible for being on the active runway causing the go around. There were at the time state vehicles; as well as maintenance vehicles; and emergency vehicles on the airport movement area at the time of the occurrence. Several vehicles were asked for their position location at the time and no vehicles claim to have been on the runway at any time. I recommend that all vehicles prior to entering the active movement area be properly equipped with radios to communicate with the appropriate controllers at all times. I recommend that all of the emergency military vehicles and their approved drivers be made properly aware of the procedures while on the active movement area and know where those areas are. I believe that some drivers are not even aware of their surrounding and unknowingly are driving in to restricted areas without proper authorization. While in very few circumstances; I believe there are times when vehicles unfamiliar with the area and unfamiliar with the rules proceed haphazardly on to the airport movement areas and then can find themselves in a position which can be very hazardous; and at the least extremely unsafe.